Page 223 - Geothermal Energy Systems Exploration, Development, and Utilization

4.7 Verification of Treatment Success 199
information on reservoir processes. Depending upon the tracer test methodology,
that is, whether continuous tracer injection or slug injection is used and whether
the test is backflow or inter-well, useful information on transport properties and
hydraulic connections after hydraulic stimulation essential for heat exchange or for
fluid re-injection in geothermal reservoirs can be obtained from the data collected
during such tests after their interpretation and modeling (Sanjuan et al., 2006;
Ghergut et al., 2007).
As the physicochemical behavior of the tracers under given reservoir conditions
(high-salinity fluid, very low redox potential, low pH, etc.) is not always well known,
the use of a minimum of two tracers (or comparison with a natural tracer or
laboratory experiments) is recommended.
Following the required information, several types of modeling approaches can be
used to analyze the tracer Return-Curve data from a slug injection: signal processing
codes such TEMPO based on a model of dispersive transfer (Sanjuan et al., 2006)
or using the moment analysis method (Shook, 2005), hydraulic or hydrodynamic
codes such as SHEMAT (Blumenthal et al., 2007) or TOUGH2 (Pruess, O’Sullivan,
and Kennedy, 2000), coupled hydro-mechanical codes, and so on.
Among the tracers recommended in the literature for use at high temperature
conditions, we can distinguish the following compounds:
• Liquid phase tracers:
– naphthalene (di, tri)sulfonates (nds, nts, ns) family: 1,5-, 1,6-, 2,6-, 2,7-nds,
1,3,5- and 1,3,6-nts, 1- and 2-ns (Rose, Benoit, and Kilbourn, 2003);
– aromatic compounds: sodium benzoate (Adams et al., 1992);
◦
–fluorescein (T < 260 C; the other organic dyes are not recommended).
The gas tracer SF 6 is little used. The use of inorganic and radioactive tracers is
limited because of the high natural background of the halides (Cl, Br, I .. . )and
difficulty in obtaining permits for radioactive tracers.
• Vapor or two-phase tracers
– Alcohols (isopropanol, butan-2-ol, etc.) and hydrofluorocarbons (volatile low
molecular weight compounds R-134a (CF3CH2F) and R-23 (CHF3), Adams
et al. (2001) as geothermal vapor-phase tracers;
– homologous series of short-chain aliphatic alcohols as geothermal two-phase
tracers: ethanol, n-propanol (Adams et al., 2004; Mella et al., 2006a, b).
The analytical methods usually used for each of these recommended compounds
are reported below:
• naphthalene sulfonates and fluorescein: HPLC with Fluorescence Detection or
spectrofluorimeter (for fluorescein only);
• sodium benzoate: extraction procedure coupled with HPLC with UV detection;
• SF 6 : Fourier transformed infra red (FTIR)/gas chromatography with electron-
capture detection (ECD);
• hydrofluorocarbons: enrichment procedure coupled with gas chromatography;
• ethanol, n-propanol: Gas chromatography with a flame ionization detection (FID).
A new method of analysis for alcohol tracers using solid phase microextraction